I I 8 NERVOUS SYSTEM 



time three lobes, which afterwards coalesce, on each side of the 

 mouth. This is in accordance with the view formulated by 

 Yiallanes l to the effect that this great nerve-centre, or brain, 

 as it is frequently called, consists essentially of three parts, viz. 

 a Proto-, a Deuto-, and a Trito-cerebron. It is, however, only 

 proper to say that though the brain and the ventral chain of 

 ganglia may appear to be one system, and in the early embryonic 

 condition to be actually continuous, these points cannot be con- 

 sidered to be fully established. Dr. L. Will has informed us 2 

 that in Aphididae the brain has a separate origin, and is only 

 subsequently united with the ganglioiiic chain. Some authorities 

 say that in the early condition the sub-oesophageal ganglion is 

 formed from two, and the supra-oesophageal from the same number 

 of ganglia ; the division in that case being 2 and 2, not 3 and 1, as 

 Viallanes' views would suggest. The inquiries that are necessary 

 to establish such points involve very complex and delicate in- 

 vestigations, so that it is not a matter of surprise that it cannot 

 yet be said whether each of these views may be in certain cases 

 correct. The supra- and sub-oesophageal ganglia are always 

 intimately connected by a commissure on each side of the 

 oesophagus ; when very closely approximated they look like one 

 mass through which passes the oesophagus (Fig. 66, A). The 

 large supra-oesophageal ganglion supplies the great nerves of 

 the cephalic sense-organs, while the smaller sub-oesophageal 

 centre gives off the nerves to the parts of the mouth. From 

 the lower and anterior part of the supra-oesophageal ganglion a 

 nervous filament extends as a ring round the anterior part of the 

 oesophagus, and supplies a nerve to the upper lip. 3 This structure 

 is not very well known, and has been chiefly studied by Lienard, 4 

 who considers that it will prove to be present in all Insects. 



Whether the two cephalic ganglia be considered as really part 

 of a single great ganglionic chain, or the reverse, they are at any 

 rate always intimately connected with the ventral ganglia. We 

 have already stated that the two cephalic masses are themselves 

 closely approximated in many Insects, and may add that in 

 some Hemiptera the first thoracic ganglion of the ventral chain 

 is amalgamated into one body with the sub-oesophageal ganglion, 



1 Bull. Soc. Philom. Paris (7), xi. 1887, p. 119, etc., and C. E. civ. 1887, p. 444. 



2 Zool. JahrbucJi. Anat. iii. 1888, p. 276. 3 Kolbe, Einfiihrung, 1893, p. 411. 



4 Arch, de Biol. i. 1880, p. 381. 



